ERNST RUSKA Max-Eyth-Strasse 20, D-1000 BERLIN 33

THE DEVELOPMENT OF THE ELECTRON MICROSCOPE AND OF ELECTRON MICROSCOPY Nobel lecture, December 8, 1986 by ERNST RUSKA Max-Eyth-Strasse 20, D-1000 BERLIN 33 A. Parents’ house, family A month ago, the Nobel Foundation sent me its yearbook of 1985. From it I learnt that many Nobel lectures are downright scientific lectures, interspersed with curves, synoptic tables and quotations. I am somewhat reluctant to give here such a lecture on something that can be looked up in any modern schoolbook on physics. I will therefore not so much report here on physical and technical details and their connections but rather on the human experiences - some joyful events and many disappointments which had not been spared me and my colleagues on our way to the final breakthrough. This is not meant to be a complaint though; I rather feel that such experiences of scientists in quest of new approaches are absolutely understandable, or even normal. In such a representation I must, of course, consider the influence of my environment, in particular of my family. There have already been some scientists in my family: My father, Julius Ruska, was a historian of sciences in Heidelberg and Berlin; my uncle, Max Wolf, astronomer in Heidelberg; his assistant, a former pupil of my father and my godfather, August Kopff, Direc- tor of the Institute for astronomical calculation of the former Friedrich-Wil- helm University in Berlin. A cousin of my mother, Alfred Hoche, was Professor for Psychiatry in Freiburg/Breisgau; my grandfather from my mother’s side, Adalbert Merx, theologian in and Heidelberg. My parents lived in Heidelberg and had seven children. I was the fifth, my brother Helmut the sixth. To him I had particularly close and friendly relations as long as I can remember. Early, optical instruments made a strong impression on us. Several times Uncle Max had shown us the telescopes at the observatory on the Königstuhl near Heidelberg headed by him. With the light microscope as well we soon had impressive, yet contradictory, relations. In the second floor of our house, my father had two study rooms connected by a broad sliding door which usually was open. One room he used for his scientific historical studies relating to classical philology, the other for his scientific interests, in particular mineralogy, botany and zoology. When our games with neighbours’ kids in front of the house became too noisy, he would knock at the window panes. This 356 Physics 1986 usually only having a brief effect, he soon knocked a second time, this time considerably louder. At the third knock, Helmut and I had to come to his room and sit still on a low wooden stool, dos à dos, up to one hour at 2 m distance from his desk. While doing so we would see on a table in the other room the pretty yellowish wooden box that housed my father’s big Zeiss microscope, which we were strictly forbidden to touch. He sometimes demonstrated to us interesting objects under the microscope, it is true; for good reasons, however, he feared that childrens’ hands would damage the objective or the specimen by clumsy manipulation of the coarse and line drive. Thus, our first relation to the value of microscopy was not solely positive. B. School, vocational choice Much more positive was, several years later, the excellent biology instruction my brother had through his teacher Adolf Leiber and the very thorough teaching I received through my teacher Karl Reinig. To my great pleasure I recently read an impressive report on Reinig’s personality in the Memoirs of a two-years-older student at my school, the later theoretical physicist Walter Elsasser. Even today I remember the profound impression Reinig’s comments made upon me when he explained that the movement of electrons in an electrostatic field followed the same laws as the movement of inert mass in gravitational fields. He even tried to explain to us the limitation of microscopi- cal resolution due to the wavelength of light. I certainly did not clearly understand all this then, because soon after that on one of our many walks through the woods around Heidelberg I had a long discussion on that subject with my brother Helmut, who already showed an inclination to medicine, and my classmate Karl who later studied medicine as well. In our College (Humanistisches Gymnasium), we had up to 17 hours of Latin, Greek and French per week. In contrast to my father, who was extremely gifted for languages, I produced only very poor results in this field. My father, at that time teacher at the same school, daily learnt about my minus efforts from his colleagues and blamed me for being too lazy, so that I had some sorrowful school years. My Greek teacher, a fellow student of my father, had a more realistic view of things: He gave me for my confirmation the book “Hinter Pflug und Schraubstock” (Behind plow and vise) by the Swabian “poet” engineer Max Eyth (1836-1906). I had always been fascinated by technical progress; in particular I was later interested in the development of aeronautics, the construction of airships and air planes. The impressive book of Max Eyth definitely prompted me to study engineering. My father, having studied sciences at the universities of Berlin and Heidelberg, obviously regard- ed study at a Technical High School as not being adequate and offered me one physics semester at a university. I had, however, the strong feeling that engineering was more to my liking and refused. C. The cathode-ray oscillograph and the short coil After I had studied two years electrotechnical engineering in Munich, my father received a call to become head of a newly founded Institute for the E. Ruska 357 History of Sciences in Berlin in 1927. Thus, after my pre-examination in Munich I came to Berlin for the second half of my studies. Here I specialized in high-voltage techniques and electrical plants and heard, among others, the lectures of Professor Adolf Matthias. At the end of the summer term in 1928 he told us about his plan of setting up a small group of people to develop from the Braun tube an efficient cathode-ray oscillograph for the measurement of very fast electrical processes in power stations and on open-air high-voltage trans- mission lines. Perhaps with the memory of my physics school lesson in the back of my head, I immediately volunteered for this task and became the youngest collaborator of the group, which was headed by Dr. Ing. Max Knoll. My first attempts with experimental work had been made in the practical physics course at the Technical High School in Munich under Professor Jonathan Zenneck, and now in the group of Max Knoll. As a newcomer I was first entrusted with some vacuum-technical problems which were important to all of us. Through the personality of Max Knoll, there was a companionable rela- tionship in the group, and at our communal afternoon coffee with him the scientific day-to-day-problems of each member of the group were openly dis- cussed. As I did not dislike calculations, and our common aim was the development of cathode-ray oscillographs for a desired measuring capability, I wanted to devise a suitable method of dimensioning such cathode-ray oscillographs in my “Studienarbeit” - a prerequisite for being allowed to proceed to the Diploma examination. The most important parameters for accuracy of measurement and writing speed af cathode-ray oscillographs are the diameter of the writing spot and its energy density. To produce small and bright writing spots, the electron beams emerging divergently from the cathode had to be concentrated in a small writing spot on the fluorescent screen of the cathode-ray oscillograph. For this, already Rankin in 1905 [1] used a short dc-fed coil, as had been used by earlier experimentalists with electron beams (formerly called “glow” or “cathode rays”). Even before that, Hittorf (1869) and Birkeland (1896) used the rota- tionally symmetric field lying in front of a cylindrical magnet pole for focussing cathode rays. A more precise idea of the effect of the axially symmetric, i.e. inhomogeneous magnet field of such poles or coils on the electron bundle alongside of their axes had long been unclear. Therefore, Hans Busch [3] at Jena calculated the electron trajectories in such an electron ray bundle and found that the magnetic field of the short coil has the same effect on the electron bundle as has the convex glass lens with a defined focal length on a light bundle. The focal length of this “magnetic electron lens” can be changed continuously by means of the coil current. Busch wanted to check experimentally his theory but for reasons of time he could not carry out new experiments. He made use of the experimental results he had already obtained sixteen years previously in Gottingen. These were, however, in extremely unsatisfactory agreement with the theory. Perhaps this was the reason that Busch did not draw at least the practical conclusion from his lens theory to image some object with such a coil. In order to account more precisely for the properties of the writing spot of a cathode-ray oscillograph produced by the short coil, I checked Busch’s lens theory with a simple experimental arrangement under better, yet still inad- equate, experimental conditions (Fig. 1) and thereby found a better but still not entirely satisfactory agreement of the imaging scale with Busch’s theoretical Fig. 1: Sketch by the author (1929) of the cathode ray tube for testing the imaging properties of the non-uniform magnetic field of a short coil [4.

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